1. Field of the Invention
This invention relates to the use of an anion-deficient europium-doped barium fluorochloride and/or fluorobromide composition as the photostimulable phosphor in an X-ray image storage panel.
2. Background
Since the discovery of X-rays, permanent records of X-ray patterns have been made using a photographic emulsion. X-ray intensifying screens are generally used in conjunction with photographic film and serve to enhance the image formed on the film. Phosphors which are the active components of such screens convert X-radiation into photons which are more readily captured by a photographic emulsion to provide a permanent record. The phosphors should be good absorbers of X-ray radiation of the energy used, they should emit light strongly in the region of the spectrum to which the film is sensitive, and they should provide sharp, undistorted film images. Ideally, the emitted light is fluorescent or "prompt" emission, with little or no delayed emission, sometimes called "afterglow", "phosphorescence" or "lag".
Another method of recording images of X-ray patterns is disclosed in U.S. Pat. No. 3,859,527. A temporary storage medium, for example, a photostimulable phosphor panel, is exposed to incident X-rays and, as a result, has temporarily stored therein an image in the form of stored energy, the image being representative of the incident pattern. At some interval after exposure, a beam of visible or infrared light scans the panel to stimulate the release of the stored energy as light so that the emitted light can be detected and, for example, converted to an electrical signal which can then be processed to produce a visible image. For this purpose, the phosphor should store as much of the incident energy as possible and, therefore, emit as little radiation as possible until stimulated by the scanning beam. Ideally, there would be no prompt emission or lag, and all emission occurs upon stimulation.
Europium-doped barium fluorohalides in general, and BaFCl:Eu in particular, have been used in conventional commercial X-ray intensifying screens for some time. The art discloses various processes for preparing the phosphor, as well as the incorporation of additives into the phosphor, all aimed at improving the prompt emission and decreasing lag.
U.S. Pat. No. 3,988,252 discloses a process for making BaFCl:Eu by firing an intimately mixed, substantially stoichiometric mixture of appropriate precursors at 600.degree. C. to 950.degree. C. in a N.sub.2, CO.sub.2, CO or noble gas atmosphere, and pulverizing the resultant composite mass. Use of hydrogen in the firing step is to be avoided since it causes high lag.
U.S. Pat. No. 4,080,306 discloses a process for making BaFCl:Eu by preparing an intimate mixture of BaFCl particles having a mean spherical equivalent diameter of less than 2 .mu.m, an europium source, and a chloride flux of at least one water-soluble chloride selected from alkali and alkaline earth metal chlorides, firing the mixture in an inert atmosphere at about 550.degree. C. to about 900.degree. C., but above that necessary to form liquid flux, for a time sufficient to effect reaction and diffusion, and washing the product with water to remove the chloride flux. The chlorides used as fluxes are preferably BaCl.sub.2, MgCl.sub.2, CaCl.sub.2, SrCl.sub.2, KCl, NaCl or LiCl, and BaCl.sub.2 /KCl or BaCl.sub.2 /CaCl.sub.2 /KCl are preferred flux mixtures.
Canadian Patent No. 896,453 discloses the use, in intensifying screens, of europium-doped strontium and/or barium chlorides and/or fluorides, prepared by heating the precursors, preferably in a 5% hydrogen in nitrogen atmosphere, at 800.degree.-1200.degree. C. According to the disclosure, the resulting phosphors are such that the cation content, including the activator, is stoichiometric with respect to the anion content.
U.S. Pat. No. 3,951,848 discloses BaFCl:Eu phosphors containing a brightness enhancement additive selected from thallium, lead, and aluminum, the phosphors being fired in a nonoxidizing or slightly reducing atmosphere.
U.S. Pat. No. 4,076,897 discloses the addition of potassium or rubidium salts to a luminescent Eu-activated barium fluorohalide phosphor, at a level of at least 0.001 weight percent of the luminescent phosphor, in order to reduce delayed fluorescence, i e., lag. A preferred composition is Ba.sub.0.98 Eu.sub.0.02 FCl with 0.1 weight percent KCl. These phosphors are used in X-ray intensifying screens and are prepared by adding the potassium or rubidium salts to the other component salts of the phosphor composition, after which the resulting composition is then intimately mixed, e.g., in ball or vibrating mills. The milling step is preferably carried out in an organic solvent, although aqueous milling may be used. The resulting suspension is dried and fired at elevated temperatures, as is well-known in the art. The phosphors of the examples of the patent were prepared by firing for 15 minutes at 880.degree. C. under a nitrogen atmosphere.
A. L. N. Stevels and F. Pingault, Philips Research Reports, 30, 277 (1975), in a discussion of the preparation of BaFCl:Eu phosphors for use in X-ray intensifying screens, state that if chlorine vacancies exist, they do not increase the lag (afterglow). This has been shown by adding small amounts of KCl to BaFCl:Eu.sup.2+, with the result that the lag (afterglow) was lower, rather than higher. The presence of metal vacancies, e.g., effected by the addition of LaCl.sub.3, resulted in greatly increased lag (afterglow). A similar effect was realized by heating the BaFCl:Eu.sup.2+ in gaseous HCl diluted with nitrogen.
U.S. Pat. No. 4,239,968 discloses a method for recording a radiation image using a photostimulable phosphor of the type disclosed in U.S. Pat. No. 4,336,154, infra, the phosphor being at least one selected from alkaline earth metal fluorohalides
II FX yA wherein MII is at least one divalent (Ba.sub.1-x M.sub.x.sup.II)FX:yA wherein M.sup.II is at least one divalent metal selected from the group consisting of Mg, Ca, Sr, Zn, and Cd, X is at least one of the halogens Cl, Br, and I, A is at least one element selected from the group consisting of Eu, Tb, Ce, Tm, Dy, Pr, Ho, Nd, Yb and Er, and x and y are numbers satisfying the conditions 0.ltoreq.x.ltoreq.0.6 and 0.ltoreq.y.ltoreq.0.2, and the wavelength of the stimulation light is not less than 500 nm nor more than 1100 nm. No preparative method for the phosphor is disclosed.
U.S. Pat. No. 4,336,154 discloses a photostimulable boron-containing phosphor which provides "heretofore unattainable high luminance". The phosphor is of the formula (Ba.sub.1-x M.sub.x.sup.II)F.sub.2.aBaX.sub.2 :yEu,zB wherein M.sup.II and X are the same as in the aforesaid U.S. Pat. No. 4,239,968 except that M.sup.II can also be Be, and a, x, y and z satisfy the respective conditions 0.5.ltoreq.a.ltoreq. 1.25, 0.ltoreq.x.ltoreq.1, 10.sup.-6 .ltoreq.y.ltoreq.2x10.sup.-1, and 0.ltoreq.z.ltoreq.2x10.sup.-1. Luminances exemplified are up to 1.85 times those of boron-free phosphors at stimulating light wavelengths of 450 to 800 nm. It also is disclosed that when an ammonium halogenide (NH.sub.4 X) is used as one of the raw materials for the phosphor, there may be times when the halogen (X) is present in excess of the stoichiometric amount in the mixture of raw materials. This excess halogen (X) is expelled from the reaction system in the form of NH.sub.4 X during firing. The phosphors are prepared by heating a mixture of the raw materials (e.g., BaF.sub.2, M.sup.II F.sub.2, BaX.sub.2 or NH.sub.4 X, a europium compound and a boron compound) at 600.degree. to 1000.degree. C. for 1 to 6 hours. The heating may be carried out in air, but it is desirable to carry it out in a neutral atmosphere such as argon or nitrogen, or in a weakly reducing atmosphere such as one containing carbon, or an atmosphere of nitrogen containing a small amount of hydrogen gas.
E. Nicklaus and F. Fischer, phys. stat. sol. (b) 52, 453 (1972) discuss the formation of chlorine and fluorine F-centres by X-irradiation and the optical absorption that occurs as a result of their existence. There is no mention of Eu doping, emitted light, or photostimulable X-ray phosphors.
K. Takahashi et al., 163rd Electrochemical Society Meeting, $pring, 1983, San Francisco, Calif., Extended Abstracts, Volume 83-1 (and in J. Electrochem. Soc., 132, 1492 (1985), discuss photostimulated luminescence (PSL) and color centers in europium-doped barium fluorohalide phosphors and conclude that the PSL in these phosphors is caused by the liberation of photoelectrons trapped at halogen ion vacancies in the host crystals.
EPO Publication No. 0 083 085 discloses the preparation of radiation image storage panels containing a bivalent europium activated complex halide represented by formula BaFX-xNaX':aEu.sup.2+ wherein X and X', each designate at least one of Cl, Br and I,x is 0.ltoreq.x.ltoreq.10.sup.-1 and a is 0.ltoreq.a.ltoreq.0.2. The firing step used in preparation is conducted in a weak reducing atmosphere, such as nitrogen gas containing a small amount of hydrogen gas.
It is an object of this invention to provide a photostimulable phosphor which is useful in X-ray image storage panels. Another object is to provide X-ray image storage panels. Still another object is to provide an anion-deficient photostimulable phosphor. A further object is to provide a process for preparing such a phosphor. A still further object is to provide an anion-deficient, europium-doped barium fluorochloride and/or fluorobromide phosphor. For further comprehension of the invention and of the objects and advantages thereof, reference may be made to the following description and to the appended claims in which the various novel features of the invention are more particularly set forth.